1. Field of the Invention
The present invention relates to a process and system for removing contaminants from a flue gas. More particularly, the present invention relates to a process and system for removing mercury from a flue gas.
2. Description of the Related Art
The combustion of coal in a coal-fired boiler, such as incident to the production of steam for turbines at a power generating station or process steam at industrial plants, produces a waste gas, referred to as flue gas, containing numerous particulate and gaseous contaminants. Among these contaminants are fly ash, nitrogen oxides, sulfur oxides and the like, whose concentration in the flue gas must be reduced, in compliance with federal, state or municipal laws and ordinances, before the gas may be emitted to the atmosphere. Included among the contaminants is mercury, which is released from the coal as gaseous elemental mercury in the elevated temperature environment of the combustion process and which, as a result of cooling of the flue gas and various contaminant-reduction treatments to which the flue gas is subsequently subjected, may end up as a mix of elemental and oxidized mercury species in the flue gas.
In a typical flue gas treatment process, the flue gas from the coal-fired boiler is first subjected to fly ash removal, such as with an electrostatic precipitator or a baghouse, followed by processes for removing sulfur oxides from the flue gas (flue gas desulfurization, or FGD). A common FGD system is a so-called wet flue gas desulfurization (WFGD) method, also referred to simply as wet scrubbing, in which the flue gas enters a large vessel (spray tower or absorber) and is sprayed with a water slurry containing lime or limestone. The calcium in the slurry reacts with the sulfur oxides, most prominently SO2, to remove these oxides from the flue gas in the form of calcium sulfite or calcium sulfate. Such process produces a substantially desulfurized water-saturated flue gas for eventual emission into the atmosphere. Optionally, prior to emission into the atmosphere, the flue gas is further treated in various ways. For example, a moisture removal step, nitrogen oxide reduction treatment, or the like.
In this typical process, a small portion of the mercury in the combustion flue gas is removed in association with the fly ash during the fly ash collection step, while additional portions of mercury are removed via dissolution in the course of the wet scrubbing process. Nevertheless, the mercury removal efficiency achieved in a typical fly ash pre-removal/WFGD process for a flue gas from a coal-fired boiler is not sufficient for achieving compliance with applicable mercury removal requirements. Improvements have been attempted in which mercury complexing agents are added to the scrubbing slurry so as to promote more effective mercury removal during the WFGD process, but even here the best results obtained are not satisfactory for compliance with applicable air quality regulations.